The disclosure relates generally to synchronizing local oscillators, and more particularly to providing devices, systems, and methods, including in distributed antenna systems (DASs), to synchronize local oscillators.
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, local area wireless services (e.g., so-called “wireless fidelity” or “WiFi” systems) and wide area wireless services are being deployed in many different types of areas (e.g., coffee shops, airports, libraries, etc.). Distributed communications or antenna systems communicate with wireless devices called “clients,” “client devices,” or “wireless client devices,” which must reside within the wireless range or “cell coverage area” in order to communicate with an access point device. DASs are particularly useful to be deployed inside buildings or other indoor environments where client devices may not otherwise be able to effectively receive radio-frequency (RF) signals from a source, such as a base station for example. Example applications where DASs can be used to provide or enhance coverage for wireless services include public safety, cellular telephony, wireless local access networks (LANs), location tracking, and medical telemetry inside buildings and over campuses.
A DAS is a type of communications system that may distribute analog communications signals. In a DAS, communications signals can be distributed from a central unit (which can also be referred to as a head-end unit) to one or more remote units forming remote coverage areas. Local oscillators (LO) may be provided in communications components in a DAS to frequency down convert or up convert distributed communications signals for distribution. In this regard, FIG. 1A illustrates an exemplary DAS 10 that provides distribution of communications signals to provide communications services to coverage areas 12(1)-12(N) in the DAS 10, where N is the number of coverage areas. These communications services can include cellular services, such as a cellular service operating using the Long Term Evolution (LTE) cellular protocol, for example. The coverage areas 12(1)-12(N) may be remotely located. In this case, the remote coverage areas 12(1)-12(N) are created by and centered on remote units 14(1)-14(N) coupled to a central unit 16. The central unit 16 may be communicatively coupled to a base station 18. In this regard, the central unit 16 receives downlink communications signals 20D from the base station 18 to be distributed to the remote units 14(1)-14(N). The remote units 14(1)-14(N) are configured to receive the downlink communications signals 20D from the central unit 16 over a communications medium 22 (e.g., coaxial cable, fiber optic cable) to be distributed to the respective coverage areas 12(1)-12(N) of the remote units 14(1)-14(N). Each remote unit 14(1)-14(N) may include one or more RF transmitters/receivers (not shown) and respective antennas 24(1)-24(N) operably coupled to the RF transmitters/receivers to wirelessly distribute the communications services to client devices 26 within their respective coverage areas 12(1)-12(N). The remote units 14(1)-14(N) are also configured to receive analog uplink communications signals 20U from the client devices 26 in their respective coverage areas 12(1)-12(N) to be distributed to the base station 18.
The communications medium 22 in the DAS 10 of FIG. 1A may be limited in bandwidth capability. For example, if the communications medium 22 is CΔT5 cable, the communications medium 22 is limited to or rated for 100 Megahertz (MHz) bandwidth at 100 meters (m) distance. As such, if the downlink communications signals 20D require greater bandwidth capacity than the bandwidth capacity of the communications medium 22, the downlink communications signals 20D may be frequency down shifted by the central unit 16 to an intermediate frequency (IF) before transmission over the communications medium 22 to the one or more remote units 14(1)-14(N). The downlink communications signals 20D at the IF can then be frequency shifted back to their original frequency in the remote units 14(1)-14(N) before being transmitted over the respective antenna 24(1)-24(N).
In this regard, FIG. 1B illustrates exemplary internal components of the central unit 16 and a remote unit 14 in the DAS 10 of FIG. 1A for frequency shifting communications signals. Only downlink communications signals 20D are shown as being frequency shifted in the DAS 10 in FIG. 1A, but the uplink communications signals 20U can also be frequency shifted. The central unit 16 receives the downlink communications signals 20D and mixes the downlink communications signals 20D in mixer 28 with a frequency signal 29 generated by local oscillator 30 to generate IF signals 32D. The frequency signal 29 generated by the local oscillator 30 is controlled by a frequency signal 33 generated by a reference oscillator 34, which may be a highly stable oscillator to in turn provide for the frequency signal 29 generated by the local oscillator 30 to be highly stable. The IF signals 32D are filtered by a filter 36 and amplified by power amplifier 38 before being transmitted over the communications medium 22 to the remote units 14(1)-14(N).
To recover the original frequency of the downlink communications signals 20D at the remote unit 14 to be radiated by the antenna 24, local oscillator 40 is provided in the remote unit 14. The local oscillator 40 generates a frequency signal 41 of the same frequency as the local oscillator 30, which is mixed with the IF signal 32D in mixer 50. Thus, the mixing of the frequency signal 41 with the IF signal 32D frequency shifts the IF signals 32D back to the original frequency of the downlink communications signals 20D to recreate the downlink communications signals 20D To ensure that the frequency of the frequency signal 29 generated by the local oscillator 30 and the frequency of the frequency signal 41 generated by the local oscillator 40 are the same or substantially the same, the frequency signal 33 generated by the reference oscillator 34 is combined with the IF signals 32D in the central unit 16 by combiner 42 as a frequency tone 44 and distributed on the communications medium 22 to the remote unit 14.
In the remote unit 14, the IF signals 32D are passed through a narrow bandpass filter 46 to recover the reference oscillator 34 frequency tone 44. The local oscillator 40 in the remote unit 14 can then be synchronized to some ratio of the recovered reference oscillator 34 frequency tone 44 by employing a phase locked loop (PLL) circuit 48 or by other means such that the frequency of the frequency signal 29 generated by the local oscillator 30 and the frequency of the frequency signal 41 generated by the local oscillator 40 are the same or substantially the same. With the local oscillator 40 of the remote unit 14 synchronized to the local oscillator 30 in the central unit 16 used to create the IF signals 32D, the IF signals 32D received by the remote unit 14 can be mixed at mixer 50 with a frequency generated by local oscillator 40 to recover the downlink communications signals 20D. The downlink communications signals 20D are filtered by filter 52 and amplified by power amplifier 54 before being transmitted by antenna 24.
As transmission signals are sent over the communications medium 22 to the remote unit 14, interference can occur between the IF signals 32D and the reference oscillator 34 frequency tone 44. Therefore, a more precise bandpass filter 46 may be required to recover the reference oscillator 34 frequency tone 44. The more precise bandpass filter 46 can be expensive and/or large.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinence of any cited documents.